Hey LP,
Without commenting on anything else, using a thread of 1.75 X 20 will
give a pitch of 0.050. If the thread is a standard 60 degree, all 3
sides of the triangle formed will be the same 0.050", and the depth of
thread will be 0.050 X 0.866 = 0.0433", or a "diameter change" at the
roots of 0.0866". In your case, this is a "raise" from the 1.75 ID
of Part A to 1.8366, so you make the OD of the "screw-in" piece to be
that size (1.8366).
Taps and dies in "unusual" threads are expensive. For instance,
Wholesale Tool wants $115 for a 1.75 X 20 tap, but only $33 for a 1.75
X 10, while the norm is NC 1.75 X 5. Of course, you can't do a 1.75 X
10 with the material sizes you give, but cost wise you might like to
reconsider.
Take care.
Brian Lawson,
Bothwell, Ontario.
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On 20 Apr 2004 17:05:17 -0700,
(LPEL98)
wrote:
I have a piece of 6061 AL tube, 2"OD, 0.125" wall, 1.75" ID, 1.25"
long. Lets call it Part A. I want to machine a second piece from
1.875" OD solid round 6061AL rod that will screw into the first piece.
Call that second piece Part B. The rod, Part B will have a 1" hole
through it and will be 1" long, and will screw into the tube, Part A
to a distance of 0.25".
Two questions.
First, since Part B will be around 1.75" diameter but only screw into
Part A for 1/4", does that suggest any specific thread pitch? I
arbitratily chose a 1.75-20, thinking that a fine thread will hold it
more snugly (less movement side to side) because of the higher thread
count. I want the joint to be as secure as possible with as little
slop and play as possible.
Second, since Part A is already 1.75" ID, the threads will be cut into
that. This means the OD of my Part B will need to be 1.75" plus
whatever the thread depth of Part A is, plus/minus a tolerance value.
I haven't done threading before. Is there a standard thread depth for
a given pitch? What is it? And how much extra (if any) should I
allow for to be sure that when the parts are made, machining
tolerances don't cause them not to mate correctly?
In other words, given Part A is 1.75"ID, what should I machine the
diameter of Part B to, and what thread depth should I machine to get
them to mate cleanly without a ton of slop and side-to-side play?
Thanks!
LP